United Kingdom version
How it works
Translate a site-specific annual energy target into a rounded panel count and a transparent material-cost scenario.
This is the United Kingdom planning view, with monetary inputs and outputs in GBP. A currency label does not establish that a tax rate, construction price, coverage rule or legal limit has been verified locally. The inputs and limitations below describe the actual scope.
Input query strings and outputs
Use an input name in this page’s URL as ?name=value, and join additional inputs with &. Portable shared links may instead use the compact ?ac= state parameter.
Input query strings
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?unitSystem=Measurement systemChoice · Optional · Default: metric · Accepted values: imperial, metric
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?annualUse=Annual electricity use, kWhNumber · Required · Default: 10,000
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?offset=Target annual generation / usePercentage · Required · Default: 80
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?specificYield=Site-specific annual yield, kWh per kWNumber · Required · Default: 1,200
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?panelWatts=Panel nameplate output, WNumber · Required · Default: 420
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?panelArea=Area of one panelNumber · Required · Default: 2
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?layoutAllowance=Roof-layout allowance beyond panel areaPercentage · Required · Default: 20
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?unitRate=Material price per panelMoney · Required · Default: 0
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?laborCost=Total labor allowanceMoney · Required · Default: 0
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?deliveryCost=Delivery allowanceMoney · Required · Default: 0
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?otherCost=Other fixed costsMoney · Required · Default: 0
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?costMultiplier=Material/labor multiplierNumber · Required · Default: 1
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?spreadPercent=Low/high sensitivity around the entered totalPercentage · Required · Default: 15
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?taxRate=Explicit tax allowancePercentage · Required · Default: 0
Outputs
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panelsWhole panels to orderNumber
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installedKwInstalled nameplate capacity, kWNumber
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generationModeled annual generation, kWhNumber
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roofAreaRoof area allowanceNumber
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materialCostMaterial costCurrency · Currency: GBP
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laborCostLabor allowanceCurrency · Currency: GBP
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deliveryCostDelivery allowanceCurrency · Currency: GBP
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otherCostOther specified costsCurrency · Currency: GBP
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taxEntered tax allowanceCurrency · Currency: GBP
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lowLow cost scenarioCurrency · Currency: GBP
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midMiddle cost scenarioCurrency · Primary output · Currency: GBP
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highHigh cost scenarioCurrency · Currency: GBP
Formula
panels = ceiling(annual use × offset ÷ yield × 1,000 ÷ panel watts).
Calculations retain numeric precision internally. Displayed values are rounded for readability. Quantities that must be purchased as whole units are rounded up only where the formula explicitly requires it.
Read the inputs before calculating
- Annual electricity use, kWh: Enter the quantity described by the label.
- Target annual generation / use: Enter the quantity described by the label.
- Site-specific annual yield, kWh per kW: Enter the quantity described by the label.
- Panel nameplate output, W: Enter the quantity described by the label.
- Area of one panel: Enter the quantity described by the label. The measurement selector converts this physical quantity; it does not change the underlying scenario.
- Roof-layout allowance beyond panel area: Enter the quantity described by the label.
- Material price per panel: Zero means unpriced, not free. Enter a current, scoped supplier quote.
- Total labor allowance: Enter the quantity described by the label.
- Delivery allowance: Enter the quantity described by the label.
- Other fixed costs: Enter the quantity described by the label.
- Material/labor multiplier: Use a verified regional index only when supplied. 1 is an unadjusted scenario, not a researched index.
- Low/high sensitivity around the entered total: Enter the quantity described by the label.
- Explicit tax allowance: Enter the quantity described by the label.
Reproducible worked example
The following is a fictional arithmetic example, not a local quote, a verified claimant or a market forecast. Its values are included so the formula can be checked. The form does not automatically confirm any verification checkbox.
| Input | Example value |
|---|---|
| Annual electricity use, kWh | 10,000 |
| Target annual generation / use | 80 |
| Site-specific annual yield, kWh per kW | 1,200 |
| Panel nameplate output, W | 420 |
| Area of one panel (m²) | 2 |
| Roof-layout allowance beyond panel area | 20 |
| Material price per panel | 0 |
| Total labor allowance | 0 |
| Delivery allowance | 0 |
| Other fixed costs | 0 |
| Material/labor multiplier | 1 |
| Low/high sensitivity around the entered total | 15 |
| Explicit tax allowance | 0 |
| Output | Calculated result |
|---|---|
| Whole panels to order | 16 |
| Installed nameplate capacity, kW | 6.72 |
| Modeled annual generation, kWh | 8,064 |
| Roof area allowance | 38.4 m² |
| Material cost | GBP 0 |
| Labor allowance | GBP 0 |
| Delivery allowance | GBP 0 |
| Other specified costs | GBP 0 |
| Entered tax allowance | GBP 0 |
| Low cost scenario | GBP 0 |
| Middle cost scenario | GBP 0 |
| High cost scenario | GBP 0 |
Interpreting a changed assumption
Start with one complete scenario and change only one input when diagnosing a difference. Recheck units, the period of every rate, whether an amount is recurring or one-off, and whether a cost is already included elsewhere. A larger calculated benefit is not evidence that an assumption is more likely to occur.
Limitations
- Material is unpriced. A zero material price is not evidence that the material is free.
- Prices and the tax allowance are user-entered planning assumptions, not a verified local quote. Low/high outputs are sensitivity scenarios, not market confidence limits.
- The multiplier applies only to material and labor; delivery and fixed costs are not multiplied.
- Nameplate output is not continuous AC output. Check shading, setbacks, inverter limits and roof structure with the installer.
Frequently asked questions
What does this Solar Panel Sizing result represent?
Translate a site-specific annual energy target into a rounded panel count and a transparent material-cost scenario. It applies the displayed formula to your inputs; it does not infer omitted facts.
Are the starting amounts verified United Kingdom prices or legal rules?
No. Numeric defaults are illustrative unless an input or dated reference explicitly identifies a researched component. Currency is GBP; local quotes, eligibility and unsupported rules must be entered and verified separately.
What must I check before relying on the result?
Check the input units, supplier or authority evidence, the stated model exclusions and the period used for costs. This arithmetic does not supply missing site-specific design or local legal rules.
Can I compare or share different assumptions?
Change one assumption at a time and use the browser-local project or share controls. A shared URL contains the encoded input values; do not include information you do not wish to disclose.
Sources and reference scope
- DOE energy-use arithmetic — retrieved 2026-09-24. Energy consumption versus power; not a local tariff or an approval of this implementation.
- PVWatts: site-specific solar estimation — retrieved 2026-09-24. External site-yield model. AskCalculators uses your entered yield; it does not call or reproduce PVWatts.
- DOE insulation overview — retrieved 2026-09-24. Insulation and thermal resistance concepts; not structural or code approval.
These references support only their stated scope. They are not evidence that every local rule or price needed by the master expansion has been loaded.